You can't connect to Exchange mailboxes after you upgrade the Click-to-Run edition of Outlook 2013 to Outlook 2016

This issue occurs if you connect to an Exchange Server 2007 server or an Exchange server for which the AutoDiscover service isn't set. To work around this issue, install November 10, 2015, update for Outlook 2013 (KB3101488). This update blocks the automatic upgrade from the Click-to-Run edition of Outlook 2013 to the Click-to-Run edition of Outlook 2016 if you have certain systems configured.